I too have this problem.
SPECS:
Sub-model: iMac (27 inch, Mid 2011)
OS version: 10.13.6
Trying to update my mac to latest possible OS.. but it refuses to update? Help! A lot of what I use will no longer work because it needs to be updated to 10.14+
Thank you
Your model cannot be upgraded beyond High Sierra. You cannot install Mojave or Catalina because the hardware is not compatible. You are running the last version of macOS that is compatible with the Mid-2011 iMac.
That Mac model isn't capable of running Mojave or newer.
(182922)
iMac won't update
